This invention relates to the field of radar signal processing within a radar receiver system.
Bistatic coherent radar displays find increasing tactical application in meeting air surveillance needs where reliance upon radiation from either a cooperative or non-cooperative transmitter is appropriate. The use of coherent-upon-receive techniques and digital signal processing in receivers of this type and the ability to use non-coherent target illuminating sources operating in a variety of radar frequency bands has made equipment of this type increasingly useful in modern radar detection and tracking.
The patent art includes several examples of radar apparatus relating to the present invention, this art includes the U.S. Patent of Eichi Kiuchi et al, U.S. Pat. No. 4,222,050, which discloses a radar system wherein the radar return signal is separated into quadrature components appearing at detectors 121 and 122. These quadrature components are converted into digital samples by analog-to-digital converters 131 and 132 and then applied to a continuous discrete Fourier transform unit 133 and a post transform processing unit 134.
The Kuichi patent is principally concerned with the utilization of a fast Fourier transform circuit in the environment of a conventional radar transmitter-receiver. One of the two Fourier transform algorithms employed in the Kiuchi patent was proposed by J. W. Coolie et al in the article "An Algorithm for the Machine Calculation of Complex Fourier Series" appearing in the publication Mathematics Computation, Vol. 19, No. 90, p. 297, April 1965. The Kiuchi patent also refers to an embodiment of the Coolie algorithm described by G. C. O'Leary in a paper "Nonrecursive Digital Filter Using Cascade Fast Fourier Transformers" published in IEEE Transactions on Audio and Electro Acoustics, Volume AU-18, No. 2, June 1970. The disclosure of the Kiuchi U.S. Pat. No. 4,222,050, the Coolie article and the O'Leary paper are hereby incorporated by reference into the present specification.
The Kiuchi patent is concerned with a conventional radar arrangement including a phase quadrature synchronous detector and an analog-to-digital converter wherein the transmitter and receiver are colocated and employ a common antenna for transmitting and receiving. The Kiuchi patent is largely silent with respect to the processing performed on signals emanating from the discrete Fourier transform circuitry. Neither the Kiuchi, Coolie or O'Leary documents are concerned with a radar receiver using target illumination by a noncooperative radar transmitter.
The patent of Ilich A. Dounce et al, U.S. Pat. No. 4,021,804, discloses a coherent timing system for a coherent-on-receive radar wherein a closed-loop circuit arrangement involving digital coded phase signals is employed to maintain coherence of the received signal. The Dounce apparatus is principally concerned with the closed-loop arrangement for maintaining coherence and provides only an output signal for further processing and use. The Dounce apparatus is also concerned with a radar apparatus having commonly located transmitter and receiver units and a single antenna for transmission and receiving.
The patent of Frederick E. Churchill, U.S. Pat. No. 3,950,750 is also concerned with maintaining coherence in a radar receiver. In the Churchill apparatus maintenance of the desired coherent phase relationships allows reduction of the amount of clutter appearing in the radar output signal. The Churchill apparatus employs a test signal that is periodically introduced into the quadrature phase detector of the radar system and uses the system response to this test signal to correct the return signals during normal radar operation. The Churchill apparatus mentions the use of a Fourier transform compensator following the phase detector or synchronous detector apparatus and discloses details of the Fourier transform output signal and its use in the correction process. The Churchill apparatus is silent with respect to contemplated use of the disclosed invention with a combination transmitter and receiver radar apparatus or a receive-only apparatus. Additionally, no discussion concerning use of the invention in a receiver intended for use with targets receiving illumination from a non-cooperative radar is included. The Churchill apparatus is also minimally concerned with the processing employed after the Fourier transform is accomplished. This disclosure of the Churchill patent is hereby incorporated by reference into the present specification.
The patent of H. J. Bose, U.S. Pat. No. 3,725,923, discloses a coherent pulsed doppler radar system which uses digital signal processing, a synchronous phase detector and Fourier transformation of memory data representing the past history of a range bin, that is, consideration of the signal contents of identically located range bins from a plurality of previous scans.
The Bose patent contemplates the use of a radar transmitter providing non-equidistant pulses but of the coherent type. The patent is silent with respect to use of the disclosed system in a non-cooperative illuminating radar and distantly located passive receiver environment. The Bose patent additionally employs threshold level processing following the Fourier transform and uses a spectrum display of the transformed signal.